Japanese Unexamined Patent Application Publication No. 2011-031363 discloses a power tool having a housing space for a driving mechanism with lubricant, a passage for adjusting a pressure inside the housing space, and a filter chamber arranged in the passage, the filter chamber having a predetermined volume and a filter arranged therein. In a state that an inside air is exhausted in conjunction with the lubricant through the passage to the outside of the housing space while the pressure inside the housing space is increased, the lubricant is caught by the filter thereby the lubricant is prevented from out flowing to the outside of the housing space.